brake noise
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Brake Noise

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Nothing grates on a persons nerves more than squealing brakes. The high pitched squeal often has the same effect as dragging fingernails across a chalkboard. American drivers will not tolerate that kind of noise from their brakes. They will ignore a Check Engine lamp and other kinds of problems for thousands of miles, but they will not put up with squeals when they step on the brake pedal.

Europeans are said to be more tolerant of brake noise. Europeans are more tolerant of a lot of things. But hey, this is America and we have to cater to the needs and wants of American drivers. What North American drivers want are safe brakes, affordable prices for brake work, and NO NOISE! Not a peep!



Of all the various complaints motorists might have after a brake job, noise is number one.

ROOT CAUSE OF BRAKE NOISE

The root cause of brake squeal is high frequency vibration. When the brakes are applied and the pads contact the rotors, tiny surface irregularities in the rotors act like speed bumps causing the pads to jump and skip as they scrape against the rotors. This, in turn, causes the pads to shake and vibrate in the calipers and against the caliper pistons. It also causes the calipers to shake and vibrate on their mounts and bushings. The greater the play between all of these parts, the greater the amplitude of the vibrations and the louder the squeal.

disc brake rotor

disc brake rotor

The rhythmic vibrations of the pads rubbing against the rotors also creates harmonic vibrations in the rotors that cause them to ring like a cymbal. The areas generating the most noise can be seen with special laboratory equipment that uses a laser to scan the surface. Changes in color reveal the intensity and frequency of the vibrations. Researchers have found that rotor vibrations are not uniform all the way around a noisy rotor. The rotor has certain spots or "nodes" that oscillate more than other areas. By redesigning the casting and changing the location of the cooling fins between the rotor faces, some of this noise can be tuned out (a good reason to use replacement rotors that have the same cooling configuration as the original).



Even the metallurgy of the rotors makes a difference. Some grades of cast iron are quieter than others. That is one of the reasons why composite rotors have been used on various vehicles over the years. Besides being lighter, composite rotors can also be quieter if the right grade of cast iron is used for the rotor disk. Replacing a composite rotor with a solid cast rotor changes the harmonics and frequency of the brake system, which may increase brake noise on some applications.

Rotor finish also affects noise. The smoother and flatter the surface, the less the likelihood of the pads chattering and dancing as they ride across the surface. Rotors should be resurfaced at the proper speed and feed rate, and with sharp tool bits to achieve the smoothest possible finish. Light sanding with an abrasive disk or flexible honing brush after the rotors have been turned can improve the surface finish even more and provide an extra degree of assurance the rotors will remain noise-free.

disc brake noise

Equally important are the pads themselves. Some friction materials are noisier than others, just as some brands of pads are quieter than others. Hard semi-metallic pads are naturally more noise prone than softer nonasbestos organic (NAO), low metallic or ceramic-based pads. The sound control qualities of any friction material depend on the fillers, lubricants and other ingredients that go into the mix. Some manufacturers add graphite and other materials to pads to dampen noise. The latest generation of low-copper and copper-free brake pads also tend to produce less brake noise than the previous generation of friction materials.

The design of the pads also influences their noise characteristics. If the leading edge of the pads has a sharp edge, it increases the tendency to grab and bounce more than if the leading edge is chamfered. That is why many premium-grade brake pads have chamfered edges. The pads may also have a slot down the middle that helps dampen vibrations will also increasing flexibility, cooling and venting.

Some friction suppliers use "Transfer Film Technology" (TFT) to prevent noise. TFT is not a coating on the pads, but part of the friction material itself. As the pads wear, they continuously transfer a very thin film to the rotor surface. This film, which leaves a dull gray coating on the rotors, fills in tiny imperfections in the rotor surface to make it smoother and more compatible with the pads, thus eliminating squeal-producing vibrations.



STOPPING BRAKE NOISE

If your brakes are noisy, how do you get rid of the noise? You can start by examining your brakes. Worn brakes are usually noisy brakes. Play between the calipers and the mounts, missing or badly corroded shims, and/or missing, bent or broken anti-rattle clips and springs can all contribute to a noise problem. So too can rough or improperly refinished rotors and hard pads.

Shims are used in disc brakes as a dampening device between the pads and caliper and caliper pistons. Shims may be steel with, or without, a soft facing material; or they may be nothing more than soft material applied to the backs of the pads. Either way, their job is to dampen vibrations at the source.

Trouble is, over time shims deteriorate. Plain steel shims become badly corroded and may crumble into flakes after years of service. Stainless steel shims are the most durable and will last for years, but stainless is expensive. Many OEMs figure the pads will be replaced in a few years anyway so they use the cheaper plain steel shims.

Some brake pads now have integrally molded built-in shims. This eliminates the corrosion issue entirely along with the chance of someone forgetting to install new shims when the pads are replaced. DO NOT install regular shims with pads that have built-in shims, otherwise the pads may not fit the calipers.



Other mechanical devices that are often installed on original equipment brake systems include anti-rattle springs and clips. These may be attached to the caliper to help hold the pads in place, or attached between the pads. If the factory-installed anti-rattle devices are missing, broken or bent, they obviously cannot do their job. Unfortunately, many technicians overlook these parts or leave them off when replacing pads because (1) they think the parts are not really necessary, or (2) it is too much hassle to replace them if they are missing, broken or bent. Consequently, these technicians probably have a lot of comebacks because of noise. If you are replacing your own brake pads,make sure these clips are put back in (or replaced if they are missing or in poor condition).

INSTALL QUIETER BRAKE PADS

Replace your old brake pads with new quieter pads. The quietest pads are typically premium quality pads that are made with the best quality friction materials, not the cheap economy pads that use the least expensive friction materials. Make sure the pads are installed with the proper shims, and anti-rattle clips and/or springs.

LIQUID BRAKE SHIMS

One alternative to mechanical shims, clips and springs is to apply a brake noise compound to the backs of the pads before they are installed. Most of these products are a high temperature silicone-based material that forms a pliable and durable cushion on the backs of the pads. The material must be allowed to cure 30 to 60 minutes before the pads are positioned in the calipers.



BRAKE LUBRICANT

Another way to dampen noise-producing vibrations is to apply a high temperature brake lubricant to the backs of the pads and the points where the pads contact the caliper. Lubricating the caliper mounts, shims and bushings is also recommended to dampen vibrations. The lubricant acts as a cushion to dampen vibrations. It also helps the parts slide smoothly so the pads wear evenly. Uneven pad wear is a classic symptom of a floating caliper that is sticking and not centering itself over the rotor.

The key here is using the right kind of lubricant. The lubricant must be heat resistant so it won't melt and run off the pads, and it must be durable so it will provide long-lasting protection. Synthetic lubricants that are compatible with the rubber parts, application and temperature range are a good choice for this type of application.



Never use ordinary chassis or silicone grease for this purpose. Also, do not allow the lubricant to come into contact with the fronts of the pads or the rotor face. Keep it on the backs of the pads and the pad contact points.

SPRAY AWAY BRAKE NOISE

The surface of a brake rotor, even with proper refinishing, can be a rough place at the microscopic level. What seems like a flat surface to the naked eye is actually a series of sharp peaks and valleys under a microscope. And as we said earlier, the rougher the finish, the more the vibration and noise when the brakes are applied.

One way to improve the surface on the rotors is to spray on an aerosol product that contains microfine aluminum particles. According to one manufacturer who makes a spray-on rotor treatment, the particles form a molecular bond with the rotor surface. This creates a "composite" surface that allows for a more controlled burnish as the new pads seat in. The result is less vibration and no annoying noise.

The same manufacturer also says their spray-on treatment increases brake torque up to eight percent, yet makes the brakes feel smoother, so there are fewer noise problems.



Spray-on surface treatments obviously do not last forever, but they can help dampen noise for several hundred to several thousand miles depending on how much the brakes are used. Pad manufacturers say rotor treatments should not be necessary as long as rotors are properly refinished, the pads are mounted securely, shimmed and there is no excessive wear or looseness in the calipers. Even so, those who use the spray-on treatments say it does not hurt anything and it eliminates brake noise complaints.

BRAKE INSTALLATION TIPS

Rotor finish should meet or exceed OEM specifications. For most applications, that means a surface finish of 60 to 80 microinches or less. A range of 20 to 50 microinches will usually guarantee quiet operation even on vehicles that are sensitive to brake noise. Most new rotors meet these specifications and do NOT require additional resurfacing.



If you are having your old rotors resurfaced, the tool bits on the lathe should be high quality and sharp. Premium quality titanium nitride coated carbide tool bits do not cost much more than standard carbide bits, and do a better job of cutting through rust and typically last twice as long. Round bits may also give a better finish than angled bits.

Composite rotors require special care when resurfacing because they lack the rigidity of cast rotors. The rotor must be supported by large bell caps or adapters, otherwise it may flex leaving tool chatter marks on the surface. The other alternative is to use an on-car lathe to resurface composite rotors.

As for how deep and how fast to cut rotors, it all depends on their condition, size and the type of equipment. One cut saves time and can achieve a nice finish on some lathes, but not others. But if a rotor has a lot of runout in it or is deeply scored, several rough cuts may be necessary to true it before a final finish cut can be made.

If the lathe has a fixed spindle speed (100 to 150 rpm), but adjustable crossfeed, a single cut crossfeed rate of about .003 in. to .005 in. per revolution should give good results. If the rotor needs two or more cuts, the depth of the first cut can be as much as .010 in. at a crossfeed rate of up to .010 inch. But the final cut should be limited to .002 to .004 inch in depth at a feed rate of no more than .002 in. per revolution.

On lathes with adjustable spindle speeds, the spindle speed should be slowed down for larger rotors. One manufacturer suggested the following speeds: for 10-inch and smaller rotors, use 170 rpm; for 11- to 16-inch rotors, use 100 rpm; for 17-inch or larger rotors, use 60 rpm.

After rotors have been turned, they should be washed with soap and water and allowed to air dry before they go back on your vehicle. If this step is skipped, microscopic particles on the surface of the rotors can become embedded in the new pads, possibly causing pads that would otherwise be quiet to squeal.

After the pads/rotors have been installed, test drive your vehicle to verify there is no noise. If the pads are the uncured variety, make 10 to 15 normal stops from about 30 mph, allowing at least 30 seconds between stops for the brakes to cool. Avoid heavy braking or panic stops for the next 200 miles.



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